Heating control device



Aug. 4, 1959 c. A. CRAFTS HEATING CONTROL DEVICE Filed April 29, 1957INVENTOR.

Qa'lA Qaffs".

United States Patent 2,898,435 HEATING CONTROL DEVICE Cecil A. Crafts,Pasadena, Calif., assignor to Robertshaw- Fulton Controls Company,Greensburg, Pa., a corporation of Delaware Application April 29, 1957,Serial No. 655,663

7 Claims. (Cl. 219-20) This invention relates to heating control devicesand more particularly to a device wherein the heating medium consists ofa slurry of fusible material having a melting temperature of a value tobe controlled.

At the melting temperature, the material employed as the heating mediumconsists of a slurry composed partly of solid material and partly ofliquid. Sincethe melting point temperature is independent of the ratioof liquid to solid material, the slurry can be maintained at constantinternal temperature over a wide range of heat application from theheating element. Thus, when such a slurry is employed as a heatingmedium, a highly regulated temperature can be obtained.

It has been found that although the temperature of the material willremain constant during variations in the liquid-solid ratio of theslurry, the conductivity of an electrically conductive material willvary with the liquid-solid ratio. It is an object of this invention tocontrol the heat input to a slurry of conductive material employed as aheating medium by a means responsive to variations in conductivity ofthe slurry.

Another object of the invention is to measure the conductivity of theslurry by means of an inductance coil which acts as a primary winding ofa transformer, the slurry material forming a secondary winding.

Another object of the invention is to position an inductance coilforming one arm of a bridge circuit in the slurry to sense theconductivity of the same to control the condition of bridge balance.

In the preferred embodiment of the invention, an inductance coil iswound on a core and immersed in a heating medium comprising a slurry ofconductive material. The inductance coil is connected as one arm of abridge circuit which is operative to energize a heating means for theslurry according to the degree of unbalance of the bridge circuit. Theinductance coil serves as a primary winding of a transformer, and theslurry material acts as a single turn secondary winding of resistanceproportional to the conductivity of the slurry. When the conductivity ofthe slurry changes as a result of a change in the liquid-solid ratio,the resistance of the secondary winding will change and be reflected ina change in impedance of the inductance coil to vary the degree ofunbalance of the bridge circuit.

Other objects and advantages will become apparent from the followingdescription taken in connection with the accompanying drawings wherein:

Fig. l is a schematic illustration of a heating control" deviceembodying this invention; and v v Fig. 2 is a detail illustrating theconstruction of an element shown in Fig. l and illustratingschematically the operation thereof.

Referring more particularly to. Fig. 1, there is shown a heating oven orcontainer 10 which has an inner chamber or heating space 12 and anouterannular chamber 14, the outer chamber 14 being filled with anelectrically conductive material 15 which will fuse when heated to "icea predetermined melting temperature and which possesses a high latentheat of fusion.

Referring now to the control system for the oven, a primary winding 16of a transformer 18 is connected across a pair of line wires L1, L2 of asuitable source of alternating voltage. A secondary winding 20 of thetransformer 18 is connected by a pair of conductors 22, 24 to a pair ofinput terminals 26, 28, respectively, of a bridge circuit indicatedgenerally by the reference numeral 30 for impressingthe alternatingvoltage of the source on said input terminals.

The bridge circuit 30 is provided with a pair of output terminals 32, 34which are connected to the input side of a power amplifier 36 bya pairof conductors 38, 40, respectively. A heating element 42 adapted to beener gized upon unbalance of the bridge circuit is connected across theoutput of the power amplifier 36 by conductors 44, 46, the heatingelement 42 being disposed in heating relationship to the material 15.

Referring more particularly to the construction of the bridge circuit30, this circuit includes a resistor 48 connected between the terminals26, 32 to define one arm of the bridge, and a resistor 50 connectedbetween the terminals 28, 32 to define another arm of the bridge. Aninductance coil 52 is connected by a conductor53 between the terminals26, 34 and a second inductance coil 54 is connected between theterminals 28, 34 by a conductor 56. The inductance coil 54 is positionedin the chamber 14 and immersed in the material 15 to be responsive tothe liquid-solid ratio thereof as will later be apparent.

The inductance coil 52 forms a primary winding of a transformer 58having a secondary winding comprising a coil 60. A variable resistor 62is connected across the secondary winding 60. The coils 52, 54 and 60are identical and each has the same number of turns.

Referring more particularly to Fig. 2, the inductance coil 54 maybewound on a suitable toroidal core 64 and both are immersed in thematerial 15. With this arrangement, the coil 54 will act as a primarywinding of a transformer, and the material 15 will form a single tumsecondary winding of resistance proportional to the conductivity of thematerial 15. This result is illustrated schematically in Fig. 2 whereinthe single turn winding formed by the material 15 is shown in dottedlines and indicated by the reference numeral 66. The resistance 68 isshown connected in the secondary winding 66 to indicate the resistanceof the material 15.

The material 15 may be of any suitable fusible condutive material havinga high latent heat of fusion. The container 10 is preferably made ofinsulating or nonconductive material or insulated from material 15 bycoatings on the inner walls fo the chambers .12, 14.

When the material 15 is in the solid state and heat is applied thereto,there will be an increase in temperature of the material 15 until itreaches the fusion temperature. At the fusion temperature, the material15will undergo a change from a solid state to a liquid state, and duringthis change of state a slurry of the material 15 will exist composedpartly of solid material and partly of liquid material. In the slurrycondition of the material 15, the fusion temperature is independent ofthe liquid-to-solid ratio and thus the slurry can be subjected to a widerange of heat input without affecting the temperature of the material15. Accordingly, while the material 15 is in this slurry condition,variations in heat input will only effect the liquid-solid ratio. Theconductivity of the material 15 is proportional to the resistancethereof which varies with the liquid-solid ratio.

The insulating properties of the container 10 are selected so that whena predetermined liquid-solid ratio of the material 15 exists in theslurry state thereof, a conrial 15.

dition of equilibrium is established where the power input to theheating element 42 is equal to the heat loss from the material 15. Thisequilibrium point may be selected at a. point where the proportions ofliquid and solid material are equal.

- The resistances 48, 50 of the bridge 30 are standard resistors and thecircuit 30 is in balanced condition when the impedance of the coil 54equals the impedance of the coil 52. The impedance of the coil 54 varieswith the conductivity of the material 15 as will now be apparent.

As indicated in Fig. 2, the material 15 acts as a single turn secondarywinding 66 on the core 64 and has a re sistance 68 proportional to theconductivity of the mate Thus, the impedance of the coil 54 when thesame is immersed in the material 15 may be determined in the followingmanner from the formula for the impedance' of a transformer:

where the terms are as follows:

R is the resistance of the secondary turn 66 and resistance 68 12 ,15the resistance of the coil 54 w is the frequency of the source L1, L2

L is the inductance of the coil 54 N is the turns ratio or the number ofturns in coil 54.

It will be apparent that since the resistance R54 inductance L 4, turnsratio N and the frequency w remain constant, all the terms in Equation 2remain fixed except R swhich varies with the conductivity of thematerial 15. Thus, the equation may be reduced to:

BR ER, (3) CRM +D CRM +D where A, B, C, D and E are constants.

Since the only variable in the above equation is R any variation in theresistance of the material 15 will be reflected in a change in the valueof the impedance of the 'coil 54.

If the coils 52, 60 are identical to the coil 54, the value of theresistance 62 appearing across the primary coil 52 will be NR However,since the turns ratio N is unity, this resistance is merely theresistance value of resistance 62.

In the case of primary coil 54, the resistance of the material 15(winding 66 and resistance 68) which will appear across the coil 54 willbe NR Since the winding 66 has a single turn, N will not be unity inthis case.

Since the coils 52, 54 are identical, the constants A, B, C and Darrived at in Equation 1 will be the same for both coils Accordingly,the impedances of the coils 52, 54 will be equal and balanced when:

where N is the number of turns on the coil 54.

Adjustment of the variable resistance 62 will serve to change the valueof the conductivity or resistance of the material 15 required to causebridge balance.

In operation, the value of resistance '62 is adjusted to cause balanceof the bridge circuit 30 when the material 15 is in a liquid state.Therefore, the bridge circuit 30 will normally be unbalanced duringoperation at the equilibrium condition where power input to the heatingelement 42 equals the heat loss from the material 15.

When the liquid-solid ratio of the material 15 is less than the ratio atequilibrium, the impedance of coil 54 will be less than impedance ofcoil 52, and the bridge 30 will be unbalanced to a large degree. At thiscondiexceed the heat loss from the material 15. The heat thus 4 suppliedto the material in excess of the heat loss thereof will cause the slurryto become more liquid and less solid. With this resulting increase inthe liquid-solid ratio of the material 15, there will be a correspondingincrease in the resistance of the material 15 which is reflected in anincrease in impedance of the coil 54. This increase in impedance of thecoil 54 will reduce the degree of unbalance of the bridge 30, thusreducing the power input to the heating element 42. The liquid-solidratio will continue to increase until a value of the impedance of thecoil 54 is reached where the power input to the heating element 42equals the heat loss from the material 15. At this equilibriumcondition, the heat input to the material 15 will be completelydissipated in heat loss. Therefore, no further increase in liquid-solidratio will occur, and the system will remain at equilibrium.

If, under some conditions, the liquid-solid ratio should increase abovethat at the equilibrium condition, the resistance of the material 15will increase to cause an increase in impedance of the coil 54. Thisincrease in impedance of the coil will reduce the degree of unbalance ofthe bridge 30 and cause a decrease in the power input to the heatingelement 42 to a value less than the heat loss from the material 15.Under these conditions, the heat loss in excess of the heat input willcause a decrease in the liquid-solid ratio until the equilibriumcondition is re-established.

. It will be apparent that if the slurry at equilibrium condition iscomposed of nearly equal portions of solid and liquid material, theinherent balancing effect of the system will prevent the material 15from changing to either completely liquid or completely solid state. If,however, the material 15 should change to a completely solid state,there will be a marked decrease in the resistance and increase inconductivity of the material 15 to cause maximum bridge unbalance. Onthe other hand, if the material 15 should change to a completely liquidstate, the bridge circuit 30 will become balanced and the power input tothe heating element 42 will be minimum. Accordingly, the system hereindescribed will maintain the temperature in the chamber 12 substantiallyat the fusion temperature of the heating medium comprising the material15.

While only one embodiment of the invention has been herein shown anddescribed, it will be apparent to those skilled in the art that manychanges may be made in the construction and arrangement of parts withoutdeparting from the scope of the invention as defined in the appendedclaims.

I claim:

1. In a system for controlling the temperature of a space, thecombination comprising a container, a heating medium for the spacepositioned within said container comprising a fusible material,electrically operated means for heating said material to the fusiontemperature thereof to form a'slurry of said material, an impedancebridge circuit comprising a pair of resistances and a pair of inductancecoils connected as the arms of said bridge circuit, one of said coilsbeing immersed in said material whereby said one coil acts as a primarycoil of a transformer and said material acts as a secondary coil of thetransformer having a resistance proportional to the liquidsolid ratio ofsaid slurry, a third inductance coil coupled to the other of saidinductance coils of said pair and defining a transformer therewith, athird resistance connected to said third coil, said bridge circuit beingbalanced when the resistance value of said material equals the value ofsaid third resistance but operative to become unbalanced'in response toa change in the resistance value of said material, and means forenergizing said heating means upon unbalance of said bridge circuit.

2. A device'responsive to the conductivity of a material comprising abridge circuit, a transformer having a primary coil and a secondarycoil, means for connecting said primary cell as one arm of bridgecircuit,

resistance means of a predetermined value coupled to said secondarycoil, an inductance coil connected as another arm of said bridge circuitand operative to cause unbalance thereof in response to a change inimpedance of said inductance coil, a transformer core for saidinductance coil immersed in the material whereby said inductance coilacts as a primary coil and the material acts as a secondary coil havinga resistance proportional to the conductivity of the material, saidresistance being adapted to change the impedance value of saidinductance coil in response to variations in conductivity of saidmaterial to efiect unbalance of said bridge circuit, and meansresponsive to bridge circuit unbalance for varying the conductivity ofthe material.

3. A device responsive to the conductivity of a material as claimed inclaim 2 wherein said bridge circuit comprises four impedance arms havinga pair of resistance elements connected as the other two arms of saidbridge.

4. A device responsive to conductivity of a material as claimed in claim3 wherein said core is toroidal in form and has said inductance coilwound thereon.

5. In a system for controlling the temperature of a space, thecombination comprising a container, a heating medium for the spacepositioned within said container comprising a fusible material,electrically operated means for heating said material to the fusiontemperature thereof to form a slurry of said material, a normally unba1-anced impedance bridge circuit, a transformer core immersed in thematerial, an inductance coil connected as one arm of said bridge circuitand Wound on said transformer core to serve as a primary winding, saidmaterial being cooperative with said coreto form a secondary windinghaving a resistance proportional to the conductivity of said material tochange the degree of unbalance of said bridge circuit upon a change inconductivity of the material, and means responsive to unbalance of saidbridge circuit for energizing said heating means.

6. The combination of claim 5 wherein said trans former core is toroidalin form.

7. The combination of claim 6 wherein said bridge circuit comprises fourimpedance arms having a pair of resistance elements connected as theother two arms of said bridge.

References Cited in the file of this patent UNITED STATES PATENTS1,617,360 Woodson Feb. 15, 1927 1,845,241 Cooley Feb. 16, 1932 2,086,966Shrader July 13, 1937 2,429,819 Jordan Oct. 28, 1947 2,524,886 Colanderet al. Oct. 10, 1950 2,640,089 Gilbert May 26, 1953 2,819,371 Aldrich etal. Jan. 7, 1958 OTHER REFERENCES Industrial Heating; vol. XXIII; No. 7,July 1956, pp. 1460, 1462, 1464.

